1. Field of the Invention
This invention relates to a method of continuously casting lead-bearing steel and more particularly to a method of continuously casting lead-bearing steel which enables lead to be uniformly and stably incorporated into a continuously cast steel strand.
2. Description of the Prior Art
The improved machinability of free-cutting steels is the result of the addition to such steels of an element such as S, Pb or Bi. Among these elements, Pb, is found particularly difficult to incorporate into the steel uniformly and stably. This is because Pb has a high specific gravity and poor solublity in steel.
The amount of Pb required to be added to a Pb-bearing free-cutting steel falls in the range of 0.1-0.4%. In contrast, the solubility of Pb in steel is said to be 0.17% in 18Cr-8Ni stainless steel and 0.08% i n 13Cr stainless steel at 1550.degree. C. (Denki-Seiko (Electric Furnace Steel), 34(19863)2, p128), figures which show that Pb solubility is especially low in ferritic stainless steel. Because of this, it is necessary to add small particles of metallic Pb to the molten steel in excess of the soluble amount to get a dispersion.
In the past, the most commonly used method of producing lead-bearing steel has been that of adding Pb to the melt in the ladle and then casting the melting into ingots. With this method, however, the Pb undergoes gravity segregation in the ladle and, as a result, the chemical composition of the steel varies with the passage of casting time. Moreover, the distribution of lead varies between the top and bottom of the individual ingots.
On the other hand, there have been atttempts in recent years to carry out the addition of Pb by the continuous casting method. In this case, it is conceivable to add the Pb to the steel in the ladle, in the mold, or in the tundish. Each of these methods has some drawbacks.
When the addition is carried out in the ladle, the lead distribution varies between the top and bottom of the strand, similarly to what was mentioned above.
In the case of adding the lead in the mold, the added lead becomes trapped by the powder layer when passing therethrough and also escapes from the melt by evaporation. The addition yield is thus low and it is therefore difficult to realize a Pb content within the prescribed range. Another problem arises in that coarse particles of Pb formed in the mold settle out, resulting in the formation of coarse Pb grains in the strand as well as uneven lead distribution.
Where the addition is carried out in the tundish, the Pb precipitating at the bottom of the tundish is entrained by the flow of melt into the mold, as are the coarse Pb particles which settle out. Coarse grains of Pb are thus formed in the strand and the lead distribution becomes uneven.
As ways for preventing the Pb precipitated at the bottom of the tundish from being entrained by the flow of melt into the mold, Japanese unexamined Patent Publication No. 58(1983)-154446 proposes a method in while the inlet of the nozzle is positioned at a high level, which Japanese unexamined Patent Publication No. 61(1986)-144250 proposes a method wherein the precipitated Pb is recovered by being passed through porous brick provided at the bottom of the tundish, thus preventing the formation of a precipitated layer of lead at the bottom of the tundish. However, neither method is able to prevent the formation of Pb grains in the strand that is caused when sedimenting coarse Pb grains are entrained by the melt flow into the mold or to overcome the problem of uneven lead distributions.